In the nuclear reactor field there has always been a need for instruments for the mapping of thermal neturon fluxes in both power and research reactors. Heretofore, various forms of self-powered neutron detectors (SPNDs) and ion chambers haved served this need in light water reactors. Although generally acceptable for use in such environments, these devices suffer several limitations when used for reactors operating at higher temperatures. For example, they cannot be used at temperatures above about 400 C., while the operating temperature of a high temperature gas-cooled reactor (HTGR) is in the vincity of 1000 C. In the case of fission chambers, the gas seals often leak, thus making them inoperable. In the case of SPNDs, the sensitivity is typically very low and the time response is very slow - in excess of thirty seconds.
One type of solid state thermal neutron detector in the prior art is known as a "thermopile" and is typically described in Instrument Society of America, Paper No. 53-14-3, published in 1953, "Preliminary Work on a U.sup.235 Thermopile", authored by J. T. DeLorenzo (one of the present inventors) and F. R. Herold. In such a device, a differential thermocouple is constructed in which the hot junction of the thermocouple is coated with a small quantity of fissile material (such as uranium-235) and the cold junction is coated with a non-fissile non-uranium equivalent material. A group of such coated thermocouples were installed in a "can" several inches in length and about 1.5 inches in diameter. Heretofore, however, these devices have not been used extensively in nuclear reactor flux monitoring because it has not been possible to construct them with adequate reproducibility.
Accordingly, it is a primary object of the present invention to provide a thermal neutron detector that can withstand the hostile environments of high temperature nuclear reactors and can be calibrated in situ.
It is another object of the present invention to provide an inexpensive thermal neutron detector of small size for use in the mapping of thermal fluxes of high temperature reactors.
It is also an object of the present invention to provide a thermal neutron detector having sufficient accuracy in the presence of gamma rays and fast neutrons whereby the thermal neutron flux of a high temperature nuclear reactor can be mapped.
Furthermore, it is an object of the present invention to provide a solid state thermal neutron dosimeter that is fabricated using known technologies whereby reliability and uniformity are achieved.
Other objects and advantages of the present invention will become apparent upon a consideration of the following drawings and a complete description of the invention.